Paper product and method of making

ABSTRACT

Paper having improved curl and cockle properties for water based, ink jet printing applications and a method of producing them. The paper contains at least 50 pounds per ton of a material that is water soluble, that can be highly concentrated during application, has low viscosity and low hygroscopicity. The paper has a maximum Cockle Value of 0.25. Certain of the materials also provide water fastness.

FIELD

The present invention is directed to paper having reduced cockle andwater induced curl, and the method of making this paper.

BACKGROUND

Hardwood and softwood wood pulp fibers are used in the manufacture ofprinting paper and newsprint. These fibers are produced in a chemicalpulping process, either sulfate or sulfite, or in a mechanical pulpingprocess. Mechanical processes would include thermomechanical andchemithermomechanical. To form the printing paper or newsprint, thesehardwood or softwood pulp fibers and wet end chemicals are mixed withwater in the headbox of the paper machine to form a suspension of fibersand chemicals. The wet end chemicals may include fillers such as calciumcarbonate and clay. The suspension of fibers and chemicals flow from theheadbox onto a wire. The water is removed from the fibers and chemicalsby both gravity and vacuum to form a wet web of pulp fibers into whichthe chemicals are incorporated. The chemicals are throughout the sheet.The sheet may be pressed and dried to remove more water.

Starch, optical brightener additives and surface size may be placed onsurface of the sheet in a surface sizing step at the size press Some ofthe materials may enter into the web if the pressure of the nip at thepress is great enough.

Thereafter the web of fiber, wet end chemicals and other materials isdried by heat, calendered and rolled into rolls. The resulting productis referred to as an uncoated or lightly coated paper sheet or web.

The uncoated sheet may be coated in another application of one or morecoating layers placed on the sheet in an off-line coating operation. Theuncoated sheet passes through a coating station and a second dryingstation. It may pass through a second calendering operation. Theresulting product is referred to as a coated paper sheet or web.

Uncoated or coated printing paper has a basis weight of from 16 to 180pounds per 3300 square feet.

The application of high speed, variable printing is experiencingtremendous growth in the printing industry, displacing conventionaloffset printing for many applications. A digital printing technologysuch as web-fed ink jet printing presents new and different challengesfor the paper maker as the optimum surface physics and chemistry ofpaper for these printers are very different than those required forconventional offset inks.

High speed, ink jet printing is exceptionally challenging because itemploys aqueous inks and a great deal of water is placed on the papersurface during the printing process. In the process, these water basedinks may be applied at high coverage at paper speeds of 500-1,000 ft.per minute. It is difficult to completely dry the paper before the paperleaves the printer. If uncoated paper is used, the water from the inkpenetrates the sheet and disrupts the bonding between the paper fibers.This creates a deformation of the paper surface, which results inunacceptable curling, cockling, or puckering of the printed paper.

Standard desk top ink jet printers are increasing in speed and some ofthe same challenges are found when printing with these printers becauseof the water placed on the paper and the difficulty of completely dryingthe paper before it leaves the printer.

Wide printers have similar challenges because of the amount of ink andwater placed on the paper.

Because of these factors, special papers are used when the print jobrequires high levels of ink coverage. These special papers are coatedwith water-absorbent silica or swellable gel materials such as polyvinylpyrilodone, or combinations of these materials. Typically, thesematerials are applied by an off-line coating operation. The price ofthese materials and the off-line application significantly increases thecost of paper for these applications.

The optical density of the printed image is also of primary concern formany print jobs as high levels of ink are required to provide vivid,robust colors. This is known as high optical density. Uncoated papersare limited in the amount of ink they can tolerate because of theirtendency to curl and cockle. Thus more expensive coated papers aregenerally required when high optical densities are needed.

SUMMARY

The present invention is directed to an uncoated paper usable with inkhaving a water content and which has a maximum Cockle Value of 0.25. TheCockle Value is used to determine the amount of cockle or water inducedcurl in the paper. An embodiment of the invention is an uncoated paperhaving a paper basis weight of 16 to 60 pounds per 3300 square feet anda maximum Cockle Value of 0.25.

An embodiment of the invention is a paper that has been treated with atleast 50 pounds per ton of paper with a material that is capable ofbeing added at the size press, blade coater or by a spray before theheated drying section. The material is water soluble, is highlyconcentrated during application, has low viscosity and lowhygroscopicity. Water soluble means a compound that is soluble toconcentrations of 20-50% of the total weight of the solution at roomtemperature or at temperatures of 50° C. or less. Highly concentratedmeans the weight or concentration of the material is 20-50% of theweight of the solution. Low viscosity means viscosities of 200centipoises (cps) or less when the weight or concentration of thematerial is 20-50% of the weight of the solution. Low hygroscopicitymeans the dried material will take up only a small amount of water inhigh humidity conditions. Another embodiment is a paper that has beentreated with at least 75 pounds of the material per ton of paper.Another embodiment is a printing paper that has been treated with up to250 pounds of the material per ton of paper. Another embodiment is apaper that has been treated with up to 300 pounds of the material perton of paper.

In one embodiment of the invention the material is a disaccharide. Inanother embodiment of the invention the material is a monosaccharide. Inanother embodiment of the invention the material is a urea. In anotherembodiment of the invention the material is a mono-citrate ordi-citrate.

In another embodiment of the invention the material is combined withstarch, latex, polyvinyl alcohol, styrene acrylic acid or an ester andthe low viscosity of the additive can be maintained.

BRIEF SUMMARY OF THE DRAWINGS

FIGS. 1-2 are digital Images of untreated commercial paper using LANDCOLow Angle Light:

FIGS. 3-4 are digital Images of sucrose treated paper using LANDCO LowAngle Light:

FIGS. 5-10 are digital images of treated and untreated pape from thesecond side cockle test method.

FIG. 11 is a graph showing the coefficient of variation of treated anduntreated samples.

FIG. 12 is a drawing of the work station for carrying out the secondside cockle test method.

FIG. 13 is a schematic diagram of a paper machine.

DETAILED DESCRIPTION

The present invention is directed to an uncoated or lightly coated paperwhich may be used for printing on ink jet printers and which has amaximum Cockle Value of 0.25 after such printing. It is also directed toan uncoated or lightly coated wide printing paper used with ink jetprinters which has a maximum Cockle Value of 0.25.

A quantitative test has been developed to determine the curl and cockleof paper. It replaces the subjective test of viewing the paper todetermine whether there was curl and cockle and the amount of curl andcockle. This prior subjective test also determined whether a sheet ofpaper had sufficient treatment. The quantitative test is the second sidecockle test method.

The second side cockle test method is used to evaluate the amount ofcockle that an inkjet print, at an ink application level of 5.9grams/square meter, produces in the unprinted or second side of a paperprinted with a block print. The present test used a Scitex Test CockleForm Print. The unprinted side of the inkjet print is illuminated usinglow angle (15°) lighting. A digital image is made of the cockled area onthe unprinted side associated with a 3.5 by 3.5 inch half-tone printedsquare on the printed side of the sample. The image is then evaluated todetermine the amount of second side cockle.

The apparatus used for the second side cockle test method is shown inFIG. 12. It includes a test platform 10, a Kodak® megaplus 8-bit digitalcamera 12, and a Dedolight® light 14. The camera 12 is mounted abovesurface 16 of the test platform 10 and at 90° to the surface 16 of thetest platform 10. The camera is aimed directly at the center of thesurface 16 of the platform. The Dedolight light 14 is mounted at anangle of 15° to the surface 16 and also aimed at the center of thesurface 16. Mathworks, Inc. Matlab® computer software is used to analyzethe images.

The samples of paper to be tested are printed on one side with a ScitexTest Cockle Form using an inkjet printer and inkjet ink. In thefollowing tests a Hewlett Packard ink jet printer HP560C was used. Theink used was Scitex Ink 2002 and the ink application level was 5.9g/square meter. The ink should be a water based ink. The paper washandled carefully so as not to crease or wrinkle the paper becausecreases or wrinkles would be analyzed as cockle.

The settings of the camera 12 were adjusted to a pixel resolution of 100microns/pixel and an f-stop of F8. The camera control was on Fixed andthe image centering was at 127. The Dedolight light 14 was adjusted foruniform low angle lighting. All lighting was from the Delolight light14. Other room lights were turned off.

The paper sample 18 was placed on the surface 16 of the test stand 10with the unprinted side of the paper turned to the camera and facing up.The 3.5 by 3.5 inch cockle area was centered in the camera field of viewwith the light aimed at the center of the cockle area. The camera'sexposure was adjusted until the average image pixel value was 127. Theimage was collected and saved to a disk.

This process was repeated for each sample.

The images were analyzed using the Mathworks, Inc. Matlab® computersoftware. Version 6, release 13 was used. The image is read into theprogram and smoothed with a 5×5 median filter to remove high frequencynoise. The mean, standard deviation and coefficient of variation werecalculated for each row and column. The larger of the maximum rowcoefficient of variability and maximum column coefficient of variabilityis taken as the sample Cockle Value. The program is evaluating thedifferences between the light and dark areas of the image anddetermining the variability.

Cockle Value means the cockle value determined by this test.

An embodiment is an uncoated paper that has been treated with at least50 pounds per ton of paper with a material that is water soluble, ishighly concentrated during application, has low viscosity and lowhygroscopicity to reduce curl, cockle or other deformation afterprinting with high levels of ink jet ink as compared to uncoated paper.The maximum Cockle Value of the treated printing paper is 0.25. A ton isdefined here as 2000 pounds. Water soluble means a compound that issoluble to concentrations of 20-50% of the total weight of the solutionat room temperature or at temperatures of 50° C. or less. Highlyconcentrated means concentrations of 20-50% of the weight of thesolution. Low viscosity means viscosities of 200 centipoises or less atconcentrations of 20-50% of the weight of the solution. Lowhygroscopicity means the dried material will not take up water in highhumidity conditions.

In another embodiment of the invention at least 75 pounds of materialper ton of paper is used. In another embodiment of the invention as muchas 300 pounds of the material per ton of paper may be used. In anotherembodiment as much as 250 pounds of the material per ton of paper may beused.

The material is applied at the size press or the blade coater. It may beapplied using a puddle, gate roll or metered size press, or a knife orblade coater. In one embodiment the material may be applied in asolution containing at least 20% by weight of material. In anotherembodiment the material may be applied in a solution containing 20 to50% by weight of the material.

In one embodiment of the invention disaccharides may be used as thematerial. Disaccharides such as sucrose and maltose can be used. Anydisaccharide having the properties noted above can be used. Manydisaccharides have viscosities below 200 centipoise (cps.) atconcentrations 20 to 50% of the weight of the solution.

Another embodiment of the invention may use monosaccharides.Monosaccharides such as glucose or mannose can be used. Manymonosaccharides have viscosities below 200 centipoise (cps.) atconcentrations 20 to 50% of the weight of the solution.

A material such as corn syrup may also be used.

Hygroscopicities and solubilities of certain polyols are listed intable 1. From this table it can be seen that malitol, lacitolmonohydrate and erythritol have the desired characteristics. TABLE 1Solubility Degree of at 25° C. hygroscopicity Polyol g/100 g H₂O % ERH @20° C. Mannitol  22 g very low Malitol 175 g low Lacitol monohydrate 140g low Anhydrous isomalt  39 g very low Erythritol  61 g very low sucrose185 g low maltose  70 g medium

EXAMPLE 1

Paper containing sucrose, starch, and surface size:

The percentages in this example are weight percentages.

A 60 gm./m² unsized paper was used for each of the samples in thisexample.

A control sample of paper was coated in a laboratory size press withethylated starch at 12% concentration. Both side of the paper werecoated to a coat weight of 40 pounds of starch per ton of paper perside. This is typical of most uncoated paper grades (Formula I).

One sample of paper was treated in a lab size press with a solutioncontaining a concentration of 35% sucrose, 5% ethylated starch and 1%surface size (Hercules UP). Both side of the paper were coated to a coatweight of 105 pounds of material per ton of paper per side (Formula II).The amount of sucrose was about 90 pounds per ton of paper per side.

A second sample was prepared with a solution containing a concentrationof 40% sucrose, 5% ethylated starch and 1% surface size (Hercules IJP).Both sides of the paper were coated to a coat weight of 112.5 pounds ofmaterial per ton of paper per side (Formula III). The amount of sucrosewas about 98 pounds per ton of paper per side.

Each of the sheets were then dried and conditioned at 50% R.H.

One set of the sheets was printed using an HP 560 printer and ScitexHigh Speed ink jet ink. The image was a 3″×3″ square, printed at 60%density, using Corel Draw, Version 10.

The printed sheets were then placed in a darkroom, face down and viewedunder a LANDSCO triple-bulb, low angle light. The degree of curl andcockle were then visually estimated. The results are given in Table 2.100% is the base case for a starch control. The others were judgedagainst the starch control. TABLE 2 Degree of Curl/ Cockle: FormulaScitex ink Formula I 100% Formula II <10% Formula III  <5%

Two commercial paper products A and B, printed with a HP 560 printerusing Scitex high speed ink jet ink were digitally recorded with a SONYMavica digital camera, under low angle light. These photographs areFIGS. 1 and 2. The Formula II and Formula III sheets from Example 1 werealso digitally recorded with a SONY Mavica digital camera, under thesame low angle light. These photographs are FIGS. 3 and 4. FIG. 3 showsthe Formula II sheet from Example 1; FIG. 4 shows the Formula III sheetfrom Example 1. The reduction in curl and cockle with high levels ofsucrose is easily observed by comparing FIGS. 1 and 2 with FIGS. 3 and4.

Another embodiment of the invention may use urea as the material. Thismaterial has the desired characteristics.

EXAMPLE 2

60 gm./m2 unsized paper was used as the base paper for the sheets inthis example.

The percentages in this example are weight percentages.

The Formula 1 control samples from Example 1 were also used as thecontrol samples in this example.

A sample of paper was treated in a lab size press with a solutioncontaining a concentration of 40% urea, 5% ethylated starch and 1%surface size (Hercules UP). Both side of the paper were coated to a coatweight of 101.5 pounds of material per ton of paper per side (FormulaIV). The amount of urea was about 88 pounds per ton of paper per side.

The sheets were then dried and conditioned at 50% R.H.

The sheets were printed using an HP 560 printer and Scitex High Speedink jet ink. The sheets were then evaluated for curl and cockle, usingthe same technique as in Example 1.

The results are shown in Table 3. TABLE 3 Degree of Curl/ Cockle:Formula Scitex ink Formula I 100% Formula IV  <5%

Another embodiment of the invention uses a salt of citric acid as thematerial.

EXAMPLE 3

60 gm./m2 unsized paper was used as the base paper for the sheets inthis example.

The percentages in this example are weight percentages.

The Formula 1 control samples from Example 1 were also used as thecontrol samples in this example.

A sample of paper was treated in a lab size press with a solutioncontaining a concentration a 25% of the monosodium salt of citric acid(monosodium citrate), heated to 50 degrees C. Both side of the paperwere coated to a coat weight of 37.5 pounds of material per ton of paperper side (Formula V).

The sheets were then dried and conditioned at 50% R.H.

The sheets were printed using an HP 560 printer and Scitex High Speedink jet ink. The sheets were then evaluated for curl and cockle, usingthe same technique as in Example 1. The sheets were also tested forwater fastness via submersion in water for 60 seconds and the ink dyewas completely immobilized by the salt.

The results are shown in Table 4. TABLE 4 Degree of Curl/ Cockle: WaterFormula Scitex ink Fastness Formula I 100% Poor Formula V  <5% Excellent

It is believed that both the mono or di salts of citric acid providewater fastness.

Water fast means the ability of ink to remain intact when exposed towater or moisture. Water fast inks do not bleed. Water based inks mustbe treated to be water fast.

Water fastness is typically obtained with a nitrogen-containing organiccompound of a cationic nature and functions by precipitating the dye inthe ink, rendering it immobile, when exposed to moisture after printing.Unfortunately, these types of materials are incompatible with anionicfluorescent whitening agents, optical brighteners, which are typicallyapplied at the size press to brighten paper. As such, these types ofadditives reduce the overall paper brightness, often times to levelsbelow customer acceptance.

Monosodium citrate maintains the brightness of the paper while providingwater fastness.

EXAMPLE 4

Samples were also evaluated using the second side cockle test method.

The percentage shown are weight percentages.

60 gm./m2 unsized paper was used as the base paper for the sheets inthis example.

The Formula 1 control samples from Example 1 were also used as thecontrol samples in this example.

A second sample was prepared with a solution containing a concentrationof 44% sucrose, 5% ethylated starch and 1% surface size (Hercules UP).Both sides of the paper were coated to a coat weight of 105 pounds ofmaterial per ton of paper per side. The amount of sucrose was 92.4pounds per ton of paper per side.

A third sample of paper was treated in a lab size press with a solutioncontaining a concentration of 35% urea, 5% ethylated starch and 1%surface size (Hercules UP). These percentages are weight percentages.Both side of the paper were coated to a coat weight of 101.5 pounds ofmaterial per ton of paper per side. The amount of urea was about 87pounds per ton of paper per side.

A fourth sample of paper was treated in a lab size press with a solutioncontaining a concentration a 25% of the monosodium salt of citric acid,heated to 50 degrees C. This percentage is a weight percentage. Bothside of the paper were coated to a net coat weight of 37.5 pounds ofmaterial per ton of paper per side. The amount of citrate was 37.5pounds per ton of paper per side.

Two commercial papers were added to the study.

The samples were evaluated both visually and using the second sidecockle test method. The image evaluation test correlated well with thevisual observation. The results are given in Table 5 and in FIG. 11.TABLE 5 Sample Visual Subjective Cockle Formula Designation Rank ValueValue  5% Starch, 1-2 Acceptable 0.2060 44% Sucrose  5% Starch, 1-2Acceptable 0.2127 35% Urea 25% mono sodium 3 Acceptable 0.2296 salt ofcitric acid 12% Starch control 4 Unacceptable 0.2658 First Choice ™ 5Unacceptable 0.2851 CI-2000 ™ 6 Unacceptable 0.3211

It was determined that paper sheets having Cockle values of 0.25 or lesswere acceptable.

FIG. 13 is a schematic drawing of a paper machine. Wood pulp fiberfurnish and wet end chemicals are mixed with water in a headbox 20 toform a slurry. The slurry exits the headbox through a slice 22 onto awire 24. The water in the slurry drains from the wire. A vacuum chest 26is also used to draw water from the slurry to form a wet paper web. Theweb is carried through press rolls 28 and a drier 30 that removeadditional water.

Additional size press chemicals or materials are placed on the wet paperweb at the size press 32. The size press may be a horizontal type withthe rolls horizontally aligned, a vertical type with the rollsvertically aligned. The materials may be placed on the web from therolls or from a puddle between the rolls. The web may, in someinstances, be coated with material by the spraying apparatus 34. Thematerials described in the various embodiments in the presentapplication would also be applied at the size press 32 or the sprayingapparatus 34.

The paper web then passes through a drying section 36. The drying isusually done by steam heated drier cans through which the paper web isthreaded. The paper is then calendered by calender rolls 38 and rolledinto paper rolls at the winder 40. The resulting product is known asuncoated paper.

This is the product of the present invention. Additional expensiveoff-machine coatings would not be required to provide a paper that has amaximum Cockle Value of 0.25.

Those skilled in the art will note that various changes may be made inthe embodiments described herein without departing from the spirit andscope of the present invention.

1. Uncoated paper having a maximum paper basis weight of 60 pounds per3300 square feet and a maximum Cockle Value of 0.25.
 2. Uncoated papercomprising cellulosic fibers and at least 50 pounds per ton of saidpapermaking fibers of a material that is water soluble, that can behighly concentrated during application, has low viscosity and lowhygroscopicity, said paper having a maximum Cockle Value of 0.25.
 3. Theprinting paper of claim 2 in which the material is a disaccharide. 4.The printing paper of claim 3 in which the disaccharide is sucrose. 5.The printing paper of claim 3 in which the disaccharide is maltose. 6.The printing paper of claim 2 in which the material is a monosaccharide.7. The printing paper of claim 6 in which the material is glucose. 8.The printing paper of claim 6 in which the material is mannose.
 9. Theprinting paper of claim 2 in which the material is urea.
 10. Theprinting paper of claim 2 in which the material is a salt of citricacid.
 11. The printing paper of claim 10 in which the material is analkaline earth salt of citric acid.
 12. The printing paper of claim 101n which the material is a mono salt of citric acid.
 13. The printingpaper of claim 10 in which the material is a di salt of citric acid. 14.The printing paper of claim 2 in which the material is corn syrup. 15.The printing paper of claim 2 in which the material is present in therange of 50 to 300 pounds per ton of paper.
 16. The printing paper ofclaim 2 in which the material is present in the range of 75 to 250pounds per ton of paper.
 17. Paper comprising cellulosic fibers and atleast 50 pounds of a disaccharide per ton of said paper, said paperhaving a maximum Cockle Value of 0.25.
 18. Paper comprising cellulosicfibers and at least 50 pounds of a monosaccharide per ton of said paper,said paper having a maximum Cockle Value of 0.25.
 19. Paper comprisingcellulosic fibers and at least 50 pounds of corn syrup per ton of saidpaper, said paper having a maximum Cockle Value of 0.25.
 20. Papercomprising cellulosic fibers and at least 50 pounds of urea per ton ofsaid paper, said paper having a maximum Cockle Value of 0.25.
 21. Papercomprising cellulosic fibers and at least 50 pounds of a salt of citricacid per ton of paper, said paper having a maximum Cockle Value of 0.25.22. Paper comprising cellulosic fibers and at least 50 pounds of a saltof citric acid per ton of paper, said printing paper being water fast.